Study confirms raising posted speed is appropriate

December 1, 2012

If you are a consistent reader of these weekly articles, you are aware of the importance of the 85th percentile speed (the speed at or below which 85 percent of the free-flowing vehicles travel) in setting speed zones. The term "free-flowing vehicles" means that drivers are not impeded by other vehicles and, therefore, are assumed to be operating at their desired speed at the measurement location. You might remember that it is a proven fact that many drivers will drive what they perceive to be an appropriate and safe speed, given the characteristics of the street, road, or highway, and weather conditions regardless of what the posted speed limit is signed. The consensus of traffic engineers throughout the country is that the value for a speed limit should be that indicated by the 85th percentile traffic speed (to the nearest 5 mph).

With the above as background, on May 4, 2007, the state Department of Transportation did a speed survey on Finney Boulevard in the village of Malone between Woodward Street and Indian Trails Drive. The speed limit on this road at that time was 30 mph. The speed study found that, in spite of the posted speed limit, the average speed was 38.5 mph and the 85th percentile was 44 mph. Of the 200 cars surveyed, only four were traveling at the 30 mph limit or below and six vehicles were traveling at 50 mph or higher. Furthermore, 97.5 percent of the free-flowing vehicles were exceeding the 30 mph speed limit. Prudent speed limit posting should not place more than one-third of the traffic in technical speed violation.

Based on the results of this study and sound traffic engineering practice, the appropriate speed limit should be placed at 45 mph. The Traffic Safety Board, the village police department and the DOT agreed to raise the speed limit between Woodward Street and the village line from 30 to 40 mph, which, along with other safety changes between Franklin Street and Woodward Street, was done around November 2011. The 40 mph limit was selected rather than the more realistic 45 mph because of lack of sidewalks along this area, resulting in pedestrians choosing to walk along the shoulder.

Some village residents were opposed to the speed limit increase, believing that by raising the speed limit 10 mph, traffic speeds would also increase 10 mph. This reasoning is flawed.

Now let's fast-forward to Oct. 11, 2012, when, at the request of the Traffic Safety Board, the DOT performed another speed survey of the same section previously studied. Remember, the new speed limit of 40 mph had been in effect for about one year.

The new study found that the average speed increased only 2.4 mph (from 38.5 to 40.9) and the all-important 85th percentile speed increased only one mph (from 44 to 45) from the earlier study. It also found that now only 57 percent of vehicles were in technical violation (traveling at 41 mph or more), still above the recommended 33 percent but far better than the former 97.5 percent in violation.

If you're still with me, this means that raising the limit from 30 to 40 mph was appropriate, and that in so doing, the average speed did not increase by 10 mph as some people, not familiar with traffic engineering and the setting of speed limits, would think.

---

For more articles on traffic law and safety, go to the traffic safety board's website, www.franklincony.org and click on "Traffic Safety Board" under departments then look for Did You Know articles under "services."